This invention relates generally to a dental prosthesis and more particularly to a dental prosthesis that is attached to an implant in the bone of a person's jaw.
It is now common when it is desired or necessary to replace a missing tooth or teeth, that the gum is opened and an implant is imbedded in the bone structure beneath the gum. The implant is held initially by friction in a socket formed in the bone or the implant may be threaded into the bone. The gum is then closed over the implant and heals. When a proper material is used for the implant, the bone and implant grow together by a process known as osseointegration so that after several months the implant becomes a permanent part of the bone structure in the mouth. Titanium has been an effective implant material.
Many firms manufacture complete systems of dental implants and prosthetic components for subsequent attachment to the implants. In a typical construction, the implant has an axially threaded hole at its top, that is, the proximal end, near the gum surface. After the implant has integrated with the bone, the gum of the implant is opened to expose the tapped hole. Then an abutment is attached to the tapped hole of the implant and extends to a level above the gum or substantially to the gum surface. The protruding free end of the abutment is constructed for attachment of a prosthesis. For preventing rotation of the prosthesis, the protruding end of the abutment requires a non-round shape and a hexagonal protrusion has been widely used. The abutment also includes a central threaded hole concentric with the threaded hole of the implant and extending inward toward the jaw bone.
A false tooth or crown is provided with a hole therethrough, known in the art as a chimney, and a non-round recess in its base corresponds in shape to the protruding non-round cross section of the abutment. Thereby, the crown can be connected to the abutment and relative rotation between them is prevented so long as critical contours of the abutment and the recess in the crown are maintained.
To prevent the crown from lifting axially from the abutment, a final screw, sometimes known in the dental profession as a "gold screw", is passed into the chimney opening and engages the tapped hole in the implant by way of the abutment so as to hold the crown axially to the abutment and to the implant. Thus, the crown cannot rotate about the abutment or implant because it is mated with the special contours on the exposed end of the abutment. The abutment is similarly mated to the proximal or outer end of the implant. The crown cannot pull away from the abutment when the gold screw has been tightened into place.
Finally, the chimney above the gold screw is filled with a composite material that hardens and is shaped as part of the crown to look like a natural tooth.
There are many variations in construction.
In many instances the crown is attached directly to a non-round protrusion of the implant and is held directly to the implant by a gold screw without use of an intermediate abutment.
The implant is intended to be a permanent fixture in the jaw bone. The abutment and crown may be replaced if necessary due to damage or poor fit by gaining access to the screw head by way of the chimney, and backing off the screw so that the crown and abutment, if used, can be separated from the implant. Thus, repairs may be made of an abutment and crown with relative convenience.
Whereas dental implantations go back to the days of the Pharaohs in Egypt, use of titanium metal implants that integrate with the bone itself, is a recent development over the last 25 to 30 years. Data so far available on the service life of such implants, indicate that most implants can be expected to have a long service life without complications of the implant itself. However, in a significant percentage of implants, a problem arises because titanium is a relatively soft metal. An unacceptable degree of looseness often develops between the implant and the adjacent abutment or crown whereby there is relative rotation between the implant and the attached elements. This rotational latitude occurs when the contours of the protrusion at the proximal end of the implant become worn as a result of lateral forces applied on the protrusion. Such forces are transmitted from the crown as the crown interacts with adjacent teeth, and perhaps bones, during the course of chewing, biting and nervous grinding (bruxism). Tremendous rotational forces are known to act on the crown. Once there is some degree of freedom for rotation of the crown relative to the implant, the non-rounded contours of the implant protrusion tend more and more toward roundness, and permit more and more rotation. In time, the connection between the implant and the abutment and crown connected thereto does not prevent rotation and is unacceptable.
In these types of failure, for example, where the original protrusion on the implant was hexagonal in shape, it has been found that the corners of the hexagonal shape have been rounded to such a degree that it is not possible to attach a new crown or abutment and prevent rotation relative to the implant.
As stated, this type of rounding on the implant protrusion occurs during normal use of the prosthesis in biting and chewing. Also, the contours of the implant protrusion may become distorted, especially unfavorably rounded or stripped, during the process of threading the implant into the jaw bone. This occurs because the implant protrusion is the means by which the driving tool engages the implant. Therefore, when hard bone is encountered, it is possible that an implant may be damaged beyond usefulness by an ill fitting socket type wrench that engages the outer surfaces of the protrusion and is used to drive-in the implant.
This situation, though costly, is not catastrophic when initially inserting an implant in that the damaged implant may be removed and a new one may be inserted. Unfortunately, after the implant has been joined to the jaw bone by osseointegration, and after years of this condition, it is extremely difficult, costly, and the cause of considerable suffering to the patient, to attempt to remove a damaged implant and replace it with another.
What are needed are a method to rehabilitate in the mouth a damaged implant that no longer holds the crown effectively against rotation, and an improved implant that better resists rotation of an attached abutment and/or crown and thus has an extended service life.